All four features enumerated above are difficult to explain in a large
class of flavour models that do very well in explaining the flavour
features of the quark sector. In particular, models with Abelian
horizontal symmetries (Froggatt–Nielsen type [1]})
predict that, in general, \(|V_{ub}|\sim |V_{us}V_{cb}|\) , \(|V_{ij}|\gtrsim m_i/m_j\)  (\(i<j\) ) and \(V\sim \mathbf {1}\) 
[2]}, [3]}. All of these are successful
predictions. At the same time, however, these models predict
[4]} that for the neutrinos, in general,
\(|U_{ij}|^2\sim m_i/m_j\)  and \(|U_{e3}|\sim |U_{e2}U_{\mu 3}|\) , in
contradiction to, respectively, points (i) and (ii) above (and there
is no way to make \(\theta _{23}\)  parametrically close to \(\pi /4\) ). On
the other hand, there exist very specific models where these features
are related to a symmetry.
